1. Field of the Invention
The present invention relates to a semiconductor wafer formed with dot marks used in fabrication history or product control in semiconductor fabrication steps, further particularly to a semiconductor wafer having dot marks which are formed at a region least influential on reading accuracy at and after semiconductor wafer and device fabrication steps and effecting no influence on electric properties of the wafer per se and produced by laser irradiation ensuring a desired amount of information.
2. Description of the Related Art
Conventionally, in makers of wafers and devices of semiconductors, with an object of process control or production control, a surface of a wafer is marked by bar codes, characters or numerical characters as a clue for finding processing conditions, fabrication history or electric properties up to the occasion. A laser marker is frequently used as an apparatus of making these. According to marking operation by a laser marker, laser beam emitted from a laser oscillator is condensed via an optical system and at the same time, a predetermined region on a surface of a wafer is scanned with laser beam by a beam scanning apparatus to thereby locally melt the surface of the wafer to form a recessed and projected surface and inscribe desired information.
Generally, the marking region is a portion of a wafer at a vicinity of an orientation flat face or a specific wafer surface region in the case of a wafer maker and a portion of a rear face or a mounting face of the substrate constituting a chip in the case of a device maker. However, in carrying out laser marking at these marking regions, regardless of a wafer or a chip substrate, a dead space for marking is needed and the yield is deteriorated. Further, since the mark is formed by melting a portion of the wafer surface, there is a concern of producing particles in melting thereof and by forming irregularities at the surrounding of the mark, there is pointed out a drawback in which a smooth face is deteriorated when a fine pattern is formed at high accuracy.
Hence, conventionally, in order to avoid such a drawback, there have been trials of executing laser marking on an outer peripheral face (side face) of a semiconductor wafer as disclosed in, for example, Japanese Patent Laid-Open No. 23512/1984 and Japanese Patent Laid-Open No. 175154/1990. That is, according to a laser marking method disclosed in Japanese Patent Laid-Open No. 23512/1984, laser beam emitted from a laser oscillator is reflected by two sheets of mirrors orthogonal to each other and deflecting an angle of deviation in accordance with marking characters and irradiated to an outer peripheral side face of a wafer via a condensing lens to thereby continuously inscribe characters. Meanwhile, according to a laser marking method disclosed in Japanese Patent Laid-Open No. 175154/1990, after vacuuming to suck a rear face center of a semiconductor wafer by a vacuum chuck, an original point is determined by an orientation flat detecting mechanism, while rotating a pulse motor by a predetermined angle, a shutter is opened and closed in cooperation with rotation and stop of the pulse motor and laser beam emitted from a laser oscillator is condensed on an outer peripheral side face of an intermittently rotating wafer to heat a spot thereof thereby forming dot marks.
Further, other than the above-described publications, for example, according to Japanese Patent Laid-Open No. 256105/1998, a total peripheral face of a semiconductor wafer in a circular disk shape is chamfered by mirror finish and the chamfered portion is inscribed with a laser mark for determining crystal orientation and a laser mark for specification, product number or wafer ID, etc.
Meanwhile, in carrying out process control in a silicon maker, particularly in order to efficiently carry out wafer production, it is preferable to attach a number to every wafer from the following reason.
It is generally difficult to summarizingly control a plurality of sheets of wafers in a state in which the wafers are contained in wafer cassettes in polishing steps or heat treatment steps of the wafer. At every processing, the plurality of sheets of wafers contained in the wafer cassettes are discharged from the cassettes and are contained again in the cassettes after the individual processings. Further, in the discharging operation, transferring among respective steps or in the containing operation to the cassettes, cracking or chipping is liable to be produced by mechanical interference between the wafers and peripheral members. When laser marking is carried out on the peripheral side face of a semiconductor wafer as disclosed in the above-described publications, the marks are lost by cracking or chipping.
Further, semiconductor wafers need to be classified to respective grades at each occasion according to modes of causing cracking or chipping or a result of inspecting electric properties etc. inspected at every processing step. And also in the classifying operation, wrong classification is liable to cause by mistake of an operator. Further, in the classifying operation, the specification significantly differs depending on users and complicated classification is needed.
Hence, when readable marking can be inscribed on the wafer surface even after having been processed in steps, easy mistake can be prevented from causing in the classifying operation. However, when laser marking is carried out on the wafer surface, as mentioned above, there poses a problem in which particles are produced thereby, the flatness of the wafer is obliged to sacrifice or the marked portion may constitute an onset of crystal strain. Hence, conventionally, wafers are not controlled individually but controlled in a unit of lot or controlled in a unit of wafer cassette.
On the other hand, according to a size of a dot mark formed on the wafer surface by normal laser marking, a limit of its diameter is 15 xcexcm and the dot mark cannot be downsized to less than the limit. Therefore, in marking a peripheral side face of a wafer by the laser marking methods disclosed in the respective publications, mentioned above, when a desired amount of information is to be inscribed, most of the peripheral side face is needed. In order to find necessary information from the pieces of information, the necessary information needs to search by rotating the wafer. In the case of the semiconductor wafer disclosed in the above-described respective publications, in a state in which the semiconductor wafer is contained in a wafer cassette, desired laser marking cannot be inscribed and naturally, these marks cannot be read.
The invention has been carried out in order to resolve these problems and it is an object of the invention to provide a semiconductor wafer having laser marks in which specifically, at a time point as early as possible in steps of fabricating a semiconductor wafer, a writable and readable region is selected in an individual wafer in a state in which the regions are mostly difficult to lose and the wafers are contained in wafer cassettes and information such as an identification number or electric properties is written to the region to thereby enable to grasp past history by a unit of wafer at and after processing steps or semiconductor fabrication steps.
In order to achieve such an object, the inventors have carried out an investigation from various angles. First, the first angle resides in selecting a region of a semiconductor wafer in which the above-described problems are not posed even by laser marking of the semiconductor wafer and marks are difficult to lose even having been processed in various fabrication steps in fabrication steps of the wafer and fabrication steps of semiconductor. The second angle resides in whether the region is provided with an area enough for sufficiently writing a necessary amount of information. Further, the third angle resides in whether the marking region is a region capable of writing and reading marks to and from the wafer while the wafers are being contained in the wafer cassettes.
For example, according to fabrication steps of a semiconductor wafer, an ingot comprising a single crystal is sliced by a predetermined thickness, a peripheral face of the slice ingot is chamfered, successively, a surface and a rear face of the wafer is polished by lapping to a required thickness and subjected to chemical polishing, a film is formed on the rear face by heat treatment, thereafter, after having been processed by mirror finish, the wafer is subjected to intermediate inspection, is cleaned and is inspected again and epitaxial growth is carried out on the surface of an acceptable product.
It is known that in such numerous steps, a portion of the wafer particularly less influenced by polishing or chemical polishing is the peripheral side face of the wafer. Further, the wafer is transferred by being gripped by robots among the above-described respective steps. In transferring the wafer, the wafer is liable to interfere with peripheral parts and by the interference, cracking or chipping is frequently caused at a peripheral edge of the wafer including an orientation flat face. The inventors have found that a portion of the peripheral edge portion of the wafer which undergoes mechanical interference the least is an inner wall face region of a notch formed for positioning of crystal orientation or the like. Further, an advantage of the inner wall face of the notch resides in that the inner wall face hardly undergoes the influence not only by the mechanical interference but also by, for example, CMP (Chemical Mechanical Polishing). Because although a processing solution flows on the inner wall face of the notch, mechanical polishing is not carried out since no external force is operated on the inner wall face of the notch and the chemical polishing action is extremely minor.
In the case of the inner wall face of the notch, three is no influence by occurrence of particles by laser marking as described above, influence in forming patterns by irregular surfaces of laser marks and influence by crystal strain and a probability of losing marks by CMP is small. Meanwhile, a pin is inserted into the notch for determining the orientation of the wafer. Therefore, interference with the pin cannot be avoided even at the inner wall face of the notch. However, a mode of the notch is prescribed by SEMI standards and is provided with a mode shown by FIG. 2.
As can be understood from FIG. 2 drawing, a notch is opened to an outer side from a peripheral side face of a wafer having a diameter of 300 mm with an angle of 90 degree and a bottom portion thereof comprises a circular arc having a radius of curvature equal to or larger than 0.9 mm and a depth from the wafer peripheral side face is set to 1+0.25 mm. Meanwhile, the slenderest one of a diameter of a pin inserted into the notch is 3 mm and there are other pins having a diameter of 4 mm and a diameter of 5 mm. Therefore, even when the slenderest pin is inserted thereinto, the pin does not interfere with the circular arc face constituting the inner wall face of the bottom portion of the notch. The inventors have firstly paid attention to the circular arc face.
Next, the inventors have investigated on whether a region in the inner wall face of the notch which does not interfere with the pin can be increased. It has been known that according to the SEMI standards, although there is a prescription of chamfering a peripheral edge of a wafer, nothing is prescribed with regard to chamfering the inner wall face of the notch and accordingly, there is a degree of freedom in fabricating the inner wall face of the notch. It has been conceived that when a fabricated face is formed at the inner wall face of the notch for avoiding interference with the pin and the fabricated face is marked, marking can be carried out with a required amount of information. In order to prevent the marking face from interfering with the pin, the inference can be resolved by chamfering upper and lower edge line portions of the notch to thereby constitute inclined faces.
However, the inner wall face per se of the notch is a very small region and accordingly, in order to write a desired amount of information on the chamfered portion constituting a portion of the inner wall face, the mark per se needs to miniaturize. Then, simultaneously with the above-described investigation, an investigation has been carried out also on what size of a mark is necessary for marking a desired amount of information at the chamfered portion constituting a portion of the inner wall face of the notch. With regard to this point, the inventors have previously proposed a method of forming a very small dot mark having a size of 1 to 15 xcexcm by using a laser marker on a surface of a semiconductor wafer as described in Japanese Patent Application No. 323080/1997 and Japanese Patent Application No. 323081/1997.
According to the method of forming a dot mark, an energy distribution of laser beam irradiated from a laser oscillator is made uniform by a beam homogenizer and a liquid crystal mask having a maximum length of 1 dot of 50 to 2000 xcexcm is controlled to drive to thereby form a desired pattern. Meanwhile, beam profile converting means for converting an energy density distribution of beam is constituted by a dot matrix having a size the same as that of a dot matrix of the liquid crystal mask in correspondence therewith and the beam profile converting means is arranged on either of a front side and a rear side of the liquid crystal mask. The liquid crystal mask is irradiated with laser beam which has been made uniform by the beam homogenizer and laser beam having a unit of respective dot transmitting through the liquid crystal mask is made to transmit through the beam profile converting means in a desired shape of the energy density distribution, thereafter, the laser beam is made to pass through a lens unit and contracted such that a maximum sectional length of respective flux of laser beam becomes 1 to 10 xcexcm and a very small dot mark is inscribed on the surface of a semiconductor wafer. At this occasion, a fabrication depth (height) of each dot mark can be controlled to fall in a range of 0.01 to 5 xcexcm.
Meanwhile, a portion in the inner wall of the notch which is mostly suitable for writing is a linear portion and when calculated from the SEMI standards, a length of the linear portion is substantially 1 mm at maximum. When alphanumerical characters are written to the linear portion with a single font of 5xc3x979 dots according to the SEMI standards, 12 characters are needed as a minimum character number to cover a minimum amount of information. At this occasion, the characters need to write at an interval of one dot.
Accordingly, the size of 1 dot in writing the characters in 1 mm of the linear portion is as follows.
1000(xcexcm)÷{6(dots)xc3x9712(characters)}=13.89(xcexcm)
That is, the size of 1 dot is 13.89 xcexcm at maximum and the size needs to be smaller than that.
Conventionally, as described in Japanese Patent Laid-Open No. 119820/1984, the size is to a degree of 20 to 30 xcexcm at most and even recently, only a dot mark having a size of 15 xcexcm can generally be marked. Therefore, although according to the conventional general laser marking method, a dot mark having a required amount of information cannot be inscribed in the above-described very small region, as has been already described, the dot marks can be inscribed in the region according to the laser marking method by the inventors. Further, when a very small dot marks can be inscribed in the region by other marking method, the invention can be embodied naturally also by the method.
As a result of the above-described investigation, according to a first aspect of the invention, there is provided a semiconductor wafer, wherein the semiconductor wafer is formed with very small dot marks on an inner wall face of a notch formed on an outer peripheral face thereof by irradiating a laser beam having a diameter of 1 to 13 xcexcm.
Further, as a region for inscribing dot marks, as has already been described, the linear portion of the notch is most pertinent, however, the linear portion is also a region interfering with the pin. Accordingly, a portion of the inner wall face of the notch which does not interfere with the pin at all is only a circular arc face (minimum radius: 0.9 mm) at a bottom portion thereof. When characters are intended to write on the circular arc face, marking of at least about one digit can be carried out in the transverse direction and accordingly, at this region, 2D codes are effectively utilized. For example, in marking by dots each having a diameter of 5 xcexcm, 18xc3x9718 dots (corresponding to 25 characters in alphanumerical characters) constitute 90 xcexcm in the transverse direction and a variation in an inscribing depth caused by irradiating the circular arc face with laser beam from one direction is 3 xcexcm at maximum and also in this case the condition can sufficiently be covered by the above-described marking method previously proposed by the inventors.
According to a second aspect of the invention, there is provided the semiconductor wafer according to the first aspect wherein upper and lower edge line portions of the inner wall face of the notch are respectively chamfered to thereby constitute inclined faces and the dot marks are formed on the inclined faces. As has already been described, this is for avoiding interference with the pin. In the case of the invention, it is sufficient that a dimension in the height direction of the inclined face is 150 xcexcm. When a dot mark of 5 xcexcm is inscribed in a rectangular region comprising a length in the horizontal direction of 180 xcexcm and a length in the vertical direction (height direction) of 60 xcexcm, a total of a number of 12xc3x9736 dots can be inscribed and when 2D codes of data matrix is utilized, information of 31 characters enough for identifying a wafer can sufficiently be inscribed.
According to a third aspect of the invention, there is provided the semiconductor wafer according to the second aspect wherein an angle of inclination of the inclined face relative to a surface of the semiconductor wafer is equal to or smaller than 30 degree. This is prescribed to prevent chipping or cracking of the wafer or to restrain film formation residue to a minimum. Further, in the case of the inclined face at the inner wall portion of the notch as in the invention, when the angle of inclination is made larger than 30 degree, an interference is caused with a fabrication lens for marking which is arranged at a final stage and accordingly, the third aspect is provided to avoid the interference.
According to a fourth aspect of the invention, there is provided the semiconductor wafer according to the second aspect wherein a surface roughness Ra of the inclined face is equal to or smaller than 1 xcexcm. The surface roughness R is represented by a value in xcexcm calculated by the following equation when a portion having a measurement length of L is sampled from a roughness curve in a direction of its center line and the center line of the sampled portion is set to X-axis, a direction of vertical magnification is set to Y-axis and the roughness curve is represented by Y=f(X).
That is, the following equation is established.
R={integration of f(X) in X=0 to L}÷L
In recent years, there is a tendency in which a shape of a side face at a peripheral edge portion of a wafer is formed in a circular arc shape with an object of preventing chipping or cracking of the wafer. The same goes with an inner wall face of a notch which is a region constituting an object of laser marking according to the invention. When dot marks having a very small mode, as mentioned above, is formed in a range of a length of 50 xcexcm on a portion having a section in such a circular arc shape as shown by FIG. 3A, a depth of the mark varies owing to the curved face. Since a thickness of a current wafer falls in a range of 600 through 800 xcexcm, even in the case of the thickness of 600 xcexcm, when the variation is 1 xcexcm and the surface roughness R exceeds 1 xcexcm, the dot mark cannot be discriminated from recesses and projections at the surrounding. Incidentally, the depth of a dot mark formed by the above-described laser marking method developed by the inventors falls in a range of 0.01 through 5 xcexcm and accordingly, the dot mark can sufficiently be identified.
According to a fifth aspect of the invention, there is provided the semiconductor wafer according to the second aspect wherein the dot marks are formed at at least either of the upper and lower inclined faces.
In fabrication steps of a wafer, there is a step proximate to a final step in which a rear face of the wafer is significantly polished to thereby thin in a total of the wafer. An amount of polishing the rear face is about several hundreds xcexcm and in contrast thereto, an amount of polishing a surface thereof is only several xcexcm. Specifically, there is case in which a wafer having a thickness of 725 to 775 xcexcm is fabricated to a thickness of 150 to 200 xcexcm by polishing the rear face. Accordingly, in consideration of the fact that the rear face side is significantly polished, it is preferable to form dot marks on the surface side as much as possible.
Meanwhile, even on the surface side of the wafer, particularly in the case of a soft mark which is polished by the CMP processing and inscribed on the wafer surface, the inscription becomes blurred gradually by the polishing action and the dot mark cannot be read. Further, when a hard mark is formed on the rear face of the wafer, an influence is effected on the flatness of the surrounding of the mark and adverse influence is effected on the depth of focus in exposure or a location of gathering dust and dirt or a source of producing thereof seems to be constituted.
In view of these points, the invention is featured in forming dot marks on the inner wall face of the notch, as mentioned above. As described in the second aspect of the invention, the wafer is chamfered to constitute the inclined faces along the upper and lower edge line portions of the notch in order to avoid also interference with the pin and the dot marks are formed on the inclined faces. Even in the case, in consideration of the above-described reason, it is necessary to select whether the dot marks are to be formed on the inclined face of the notch on the surface side of the wafer or whether the dot marks are to be formed on the inclined face of the notch on the rear face side of the wafer, as necessary. Normally, as mentioned above, even in the case of the inclined face on the surface side, it seems that influence of polishing by the CMP processing is not considerably effected and therefore, it is sufficient to form the dot marks on the upper inclined face of the notch. However, in consideration of a rare case, it is preferable to form the same dot marks also on the lower inclined face of the notch. Naturally, according to the invention, the dot marks can also be formed only on the lower inclined face of the notch.